The invention relates to a method and an apparatus for determining the root mean square value of a vibration variable measured on a machine.
Root mean square values of vibration variables play an important role in the vibration-based state monitoring of machines, and so the reliable determination of root mean square values is an essential constituent part of such state monitoring (the root mean square (RMS) value of a signal, also referred to as effective value, is the square root of the mean power). In practice, the root mean square value of a vibration variable can be determined by virtue of measuring the vibration variable over a certain measurement duration and calculating the root mean square value yell from the measured signal, taking into account a generally unambiguously set transfer function (frequency response), according to
      y    eff    =                              1          τ                ⁢                              ∫            0            τ                    ⁢                                                                                      y                  ⁡                                      (                    t                    )                                                                              2                        ⁢            dt                                =                            1          τ                ⁢                              ∫            0            τ                    ⁢                                                                                                          g                    ⁡                                          (                      t                      )                                                        *                                      x                    ⁡                                          (                      t                      )                                                                                                  2                        ⁢            dt                              where τ denotes the signal length, g(t) denotes the transfer function and x(t) denotes the measured signal.
While the transfer function is set, as a rule, in order to keep the root mean square values comparable with one another, there usually is no strict guideline for the measurement duration. A significant influence of the measurement duration on the obtained root mean square value can emerge, in particular, in the presence of beats, i.e. if the vibration signal contains at least two substantial frequencies lying close together. In such a case, the obtained root mean square value may oscillate as a function of the measurement duration, making a reliable ascertainment of the root mean square value difficult. In particular, a root mean square value that differs from measurement to measurement may emerge as a result thereof, even though there has been no change in the machine state. By way of example, beats occur in the case of two fans, pumps or motors that rotate approximately the same or in the case of the construction-related slip of asynchronous motors and cannot, as a matter of principle, be suppressed by band-pass filters.
DE 101 34 013 A1 relates to a method for monitoring vibrations of machines, wherein the time interval between two measurement processes is set on the basis of the result of a test measurement.
WO 2011/087440 A1 describes a vibration monitoring of machines, where test measurements under laboratory conditions are used to determine the values of amplitude peaks as exactly as possible.